1. Field of the Invention
The present invention relates to methods and apparatus for forming spunbond webs and, more particularly, to formation of spunbond webs by deflection of extruded fibers onto a non-horizontal web-forming surface, such as a moving screen belt.
2. Description of the Related Art
Non-woven fabrics made from melt-spinnable polymers are commonly produced using spunbond processes. The term xe2x80x9cspunbondxe2x80x9d refers to a process of forming a non-woven fabric or web from an array of thin, melt-spun polymeric fibers or filaments produced by extruding molten polymer from orifices (the orifices can be, for example, those of a long, generally rectangular spinneret or of a plurality of spinnerets). Below the spinneret, the extruded fibers form a vertically oriented curtain of downwardly moving strands that are at least partially quenched before entering a long, slot-shaped air aspirator positioned below the spinneret. The aspirator introduces a rapid downwardly moving air stream produced by compressed air from one or more air aspirating jets. The air stream creates a drawing force on the fibers, causing them to be drawn between the spinneret and the air jet, thereby attenuating the fibers. Upon exiting at the bottom of the aspirator, the drawn fibers are randomly laid on a forming surface, such as a moving conveyor screen belt (e.g., a Fourdrinier wire), to form a continuous non-woven web of fibers. The web is subsequently bonded using one of several known techniques to form a stable, non-woven fabric. A common bonding method involves lifting the web from the moving screen belt and passing the web through two heated calender rolls. Often, one of the rolls is embossed, causing the web to be bonded in numerous spots. Carded or air-laid webs can also be formed from such polymeric fibers.
It has long been understood that the distances between the spinneret, aspirator and web forming surface are important parameters in the formation of non-woven webs, and the ability to adjust these distances is highly desirable, if not essential, in any apparatus designed to produce a variety of non-woven webs having different properties (e.g., denier, weight, texture, polymer composition, etc.). For example, in U.S. Pat. No. 3,802,817 to Matsuki et al., the disclosure of which is incorporated herein by reference in its entirety, it is recognized that good fiber formation and high production require optimization of the distance L from the spinneret to the aspirator (i.e., the xe2x80x9cspinningxe2x80x9d distance), and optimum web formation requires optimization of the distance M from the aspirator to the wire belt (i.e., the xe2x80x9claydownxe2x80x9d distance). Unless the machine is dedicated to producing only one type of fabric, these distances must be varied in accordance with the desired fiber denier, the particular polymer being spun, and the fiber cross-sectional shape. In a large production spunbond machine, the spin beam containing the spin pack assembly (of which the spinneret is the bottom element) has a weight of several tons, since the spin beam is a pressure vessel using boiling Dowtherm or a similar liquid/vapor material to provide uniform heat. The beam also contains metering pumps to control the polymer rate through the spinneret. For an apparatus making a web of three meters or more in width, the forming table, which controls and drives the Fourdrinier wire, also weighs several tons. The aspirator is not quite as heavy and is typically more compact than the beam and forming table; nevertheless, the aspirator may weigh over one ton for a three-meter machine.
Conventionally, two of the three major machine elements must be moveable in the vertical direction to enable independent adjustment of the spinning distance L and the laydown distance M. Moving the spin beam is particularly problematic, since the spin beam is fed molten polymer through heated piping from a screw extruder that also weighs several tons. If the piping is fitted with rotary joints, it is possible to raise and lower the beam while the extruder is at a fixed height; however, these rotary joints are prone to leakage when operated at 280xc2x0 C. and over 1000 psig polymer pressure, which are normal operating conditions. The extruder can be raised and lowered with the beam to allow shorter piping and no rotary joints, but then the additional several ton weight of the extruder must be lifted along with the beam.
If, instead, the spin beam remains at fixed height and the forming table and aspirator are adjusted vertically, considerable additional expense will be incurred in providing the capability to move these components, particularly the forming table. Also, if the forming table is moved vertically, independent adjustment of spinning distance L cannot be achieved on a machine with multiple beams and aspirators. Independent adjustment is desirable in any number of circumstances where plural beams and aspirators produce plural arrays of fibers. One example is in the production of a laminated fabric where, for example, there are three laydown zones and the middle zone produces fibers of much finer denier than the first and second zones, capturing a fine denier web between two coarser denier webs. Such independent adjustments cannot be achieved by raising or lowering a horizontal forming table and are very expensive if achieved by individually raising or lowering each spin beam with its related piping, pump drives and quench ducts.
Another problem encountered with spunbond processes occurs when the spinning distance L is kept particularly short in order to permit formation of fine fibers at high production speeds. U. S. Pat. No. 5,545,371 to Lu, the disclosure of which is incorporated herein by reference in its entirety, describes a process wherein the distance L is less than 50 cm, and this distance is adjusted to control the diametric size of the filaments (fiber denier). Adjusting the spinning distance L will also change laydown distance M, unless laydown distance M can be independently adjusted by raising or lowering the screen wire, as is shown in the Lu patent. When the spinning distance L is short (e.g., less than 50 cm), the process can be operated under certain conditions of polymer flow (normally expressed in grams/spinneret orifice/minute) and aspirator air velocity. However, if the polymer flow or air velocity is too high, the fibers will not cool sufficiently and will break between the spinneret and the aspirator. Each break is followed by a drip of polymer at the leading end of the new fiber that is forming from the spinneret orifice where the break occurred. If the process conditions are such that very few fiber breakages occur, economical web production can take place. On the other hand, if frequent fiber breakages occur, these drips can land on the Fourdrinier belt while still hot and become bonded to the belt as the belt passes through the compaction rolls. The belt must then be stopped and production lost in order to periodically clean the belt. Until the belt is cleaned, the drips stuck to the belt can snag fibers in the web and cause web damage.
Accordingly, there remains a need to improve upon conventional spunbond web formation techniques for adjusting the spinning and laydown distances and to mitigate the detrimental effects of polymer drips that occur during spunbond web formation.
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to provide a process and apparatus that significantly reduces the cost of spunbond machinery by eliminating the need to raise and lower the spin beam and the forming table while preserving the ability to independently adjust the spinning distance L and the laydown distance M.
Another object of the present invention is to separate a significant portion of polymer drips from the extruded fibers before the drips can land on the web-forming belt, thereby reducing belt damage and downtime resulting from polymer drips.
Yet another object of the present invention is to permit formation of a web in two or more stages from two or more spin beams and aspirators, thereby increasing the production rate of the machine.
Still another object of the present invention is to permit independent adjustment of the distance from the spinneret to the aspirator (i.e., the spinning distance) for each of multiple stages of laydown (e.g., for two separate arrays of polymers respectively extruded from two spin beams).
Another object of the invention is to provide a spunbond machine with improved operator accessibility and easier installation of spin packs.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, the aforementioned difficulties associated with independently adjusting the spinning distance L and the laydown distance M are overcome by employing novel techniques for adjusting the laydown distance M that do not involve raising or lowering the spin beam or forming table. More specifically, once an array of fibers extruded from the spinneret of a spin pack are drawn through an aspirator, the attenuated fibers discharged from the bottom of the aspirator are deflected at a significant angle with respect to vertical and deposited on a web-formation belt that is oriented non-horizontally. For example, the fibers can be deflected sideways a full 90xc2x0, causing the fibers to travel horizontally, and can subsequently be deposited on a vertically moving screen belt. Efficient deflection of the fibers can be achieved using a Coanda device that entrains the air flow exiting the aspirator along a smooth curved surface in accordance with the Coanda effect. In this arrangement, the laydown distance M between the trailing edge of the Coanda device and the web-forming belt does not lie along a vertical line, as in conventional devices. Rather, in the case of a 90xc2x0 deflection angle and a vertically oriented web-forming belt, the laydown distance M is a horizontal distance that can be modified by horizontally moving the forming table supporting the belt, by horizontally moving the aspirator, and/or by altering the distance between the belt and the trailing edge of the Coanda device (e.g., by adjusting the size or shape of the Coanda device). The spinning distance L, which is not affected by adjustment of the laydown distance M, can be independently adjusted by raising or lowering the aspirator in a conventional manner. The apparatus of the present invention thereby advantageously avoids the need to move the spin beam and the need to raise or lower the forming table, while permitting independent adjustment of the spinning distance L and the laydown distance M.
Further, a substantial portion of the heavier polymer drips that develop during fiber extrusion will not be entrained and deflected by the Coanda device and will fall vertically downward from the aspirator output into a collection trough. Consequently, these drips will not reach the web-forming belt and cause the aforementioned problems associated with polymer drips fusing to the screen belt. Accordingly, the frequency with which the belt requires cleaning is reduced and production time and efficiency are increased.
In accordance with another embodiment of the present invention, the fiber deflection technique of the present invention is applied to form a double-layer web by separately depositing two fiber arrays at two different points along the path of the web-forming belt. The belt is routed by rolls such that two different vertical surfaces are formed by the belt along its path. The above-described fiber deflection technique is employed to deposit one array of fibers onto one of the vertical belt surfaces, while a second, similar spin beam/aspirator arrangement is used to deposit another array of fibers on top of the first array of fibers at a point on the second vertical surface. In this embodiment, both the spin beams and the forming table/belt are mounted in a fixed position. Nevertheless, the spinning distances L and laydown distance M for each of the fiber arrays can be adjusted independently by moving the associated aspirator and/or by changing the size or shape of the associated Coanda device.
In accordance with another embodiment of the present invention, the fibers discharged from the aspirator are deflected at an angle of less than 90xc2x0 (but preferably at an angle of at least 45xc2x0) onto a belt traveling at an inclined angle, such that the fibers are directed substantially normal to the belt. The inclined belt can be a short belt that is separate from a main conveyor belt. Because the short inclined belt can be moved substantially independent of the main belt, this arrangement offers the advantage of greater ease and flexibility in adjusting the laydown distance M to the inclined belt. The inclined belt unit also provides greater room and flexibility for positioning quenching ducts in the vicinity of the spin beam. In a two or more layer web formation process, two or more separate short belts can respectively deliver webs to a main belt where the webs are overlaid and bonded.
While the use of a Coanda device to deflect the arrays of fibers is preferable, other mechanisms can be used to effectively deflect the fibers exiting the aspirator onto a non-horizontal belt. For example, a curved deflection plate can be placed at the output of the aspirator to direct the fibers in a sideways direction.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.